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Green roofs

Many products that aim to make the construction of green roofs simpler and more efficient are now available, including pre-seeded blankets, planted modules and drainage layers

A standard construction practice in many other countries for years, green roofs have been used for their insulative properties. Until recently they were considered a vernacular building practice, yet since the 1960s an increased concern for the environment has driven the rise in their popularity, addressingthe decline in green space that exists within urban areas.

Green roofs have significant environmental benefits. In urban areas they can reduce the urban heat island effect, which is caused by increased development. They also increase biodiversity and create habitats within our urban centres. The use of green roofs can form part of a sustainable urban drainage solution. These roofs reduce the amount of water being released into the sewer system, particularly useful during a storm situation when they slow the initial surge of stormwater runoff . For these reasons their use is increasingly being incorporated into policy. In 2008 the revised London Plan moved from encouraging green roofs to expecting them on all major developments.

There are two main types of green roof: extensive and intensive. Extensive roofs are typically 50-150mm in depth and are lightweight. They will often feature plants such as sedums, mosses and grasses. Intensive roofs are - as their name suggests - much more substantial. Their build up can total as much as 1,500mm and they usually contain a mix of hard and soft landscape, featuring trees and shrubs.

A green roof, whether intensive or extensive, will have a basic build up comprising three layers: the drainage layer, a filter layer and the vegetation layer. The depth and the specification of each of these layers will differ dependent on the plants that are to be supported.

The roof structure is an important consideration in green-roof design. A rough guide to loadings would suggest that an extensive roof can weigh anything from 60 to 150kg/m², and an intensive roof up to 500kg/m². If a green roof is part of the initial design of the building, the structural engineer will be able to take this into consideration in structural calculations from the beginning; if the green roof is added at a later stage, however, costs can rise.

Oldroyd Xv green

Oldroyd Xv green

A damp-proof membrane designed for use on green roofs. The rubberised outer layer on both sides minimises slipping of the membrane on the roof during laying, providing further grip when laying the turf or sedum.

Oldroyd Xv20 greenXtra

Oldroyd Xv20 greenXtra

Oldroyd Xv20 greenXtra is a perforated membrane with a 20mm deep stud specifically designed for green-roof use. The studs collectively form a rainwater reservoir, providing water for the roof plantings, which is useful in areas of low rainfall.

Foamglas T4

Foamglas T4

Foamglas insulation has good levels of compressive strength and resistance to moisture, which makes it particularly well suited to green-roof applications. It also resists degradation from root penetration.

Xero Flor XF300

Xero Flor XF300

For this plant matting the plants are grown on a blanket, which is harvested like turf and then laid out above the waterproofing layer. The blankets are very lightweight, easy to maintain and provide instant greening to the roof.

Floraset

Floraset

A lightweight drainage board particularly designed for roofs with little or no drainage falls. The board collects water in its storage cells, while a multidirectionalchannel system on the underside allows positive drainage of any excess.

Green Roof Cultivated Mat

Green Roof Cultivated Mat

Grown in the UK, these mats can be dispatched at short lead in times with the minimum of transportation. The sedum plants are naturally hardened to the UK environment during their initial growing process.

Biodrain Mat

Biodrain Mat

Langley’s Biodrain is produced from recycled foam with a cuspated base layer. The cuspate design allows for drainage of excess water, while the foam material provides a water retention layer, feeding the system above. The drainage layer is a recycled material free from polyurethanes.

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The Architects' Journal is the voice of architecture in Britain. We sit at the heart of the debate about British architecture and British cities, and form opinions across the whole construction industry on design-related matters